Forged piston

ABSTRACT

To provide a forged piston made of an aluminum alloy which is excellent in heat resistance and abrasion resistance and the lightening of which is enabled. A forged piston made of an aluminum alloy is provided including 8 to 18 wt % of Si, 0.5 to 3 wt % of Cu and 1 to 5 wt % of Ni, further including Al and unavoidable impurities, in which more Ni is included than Cu and the maximum length of an intermetallic compound generated by Al and Ni is 3 to 100 μm.

TECHNICAL FIELD

The present invention relates to a forged piston made of an aluminum alloy.

BACKGROUND

Typically, a piston is used in an internal combustion engine designed for converting energy produced by combustion to motive power. Improvements may yet be made in such pistons to provide a piston that is lighter and has more excellent properties in heat resistance and abrasion resistance. A piston that can meet such properties are in demand. Particularly, forged pistons made of an aluminum alloy attract attention. Past piston designs have been proposed in attempt to achieve such desirable characteristics.

For example, JP-A No. 216487/1995 proposes an aluminum alloy on which abrasion resistance is conferred by Si, which includes a precipitation hardening element of Cu and Mg and further includes Fe and Ni, elements that contribute to the enhancement in strength, and includes Mn, Ti, Zr and V provided for the effect of recovery at high temperature and the effect of inhibiting recrystallization coexist. However, as the effect of the enhancement of the strength in this case is acquired only approximately at 250° C. at the highest, sufficient strength at the top of a piston at high temperature cannot be acquired in the case where the piston is made of such an aluminum alloy.

In the meantime, JP-A No. 54053/2000 proposes an aluminum alloy in which the maximum length of a crystallized substance by casting is controlled to be 200 μm and proposes that the strength of which at further higher temperature is enhanced by strengthening the dispersion of the crystallized substances. However, in this reference, as a transition element for crystallizing an intermetallic compound is added in a large quantity, the specific gravity increases and the enhancement of engine performance by lightening is hindered. Besides, as a bulky crystallized substance causes an internal defect and a crack in forging, difficulties arise for enhancing the quality of material by forging becomes difficult.

In the meantime, JP-A No. 260089/1996 proposes that the strength at high temperature equal to or exceeding 250° C. is enhanced by setting the percentage content of Ni, out of the elements of an Al—Si aluminum alloy used in a piston, to 2 to 6 wt %. However, in this reference, as Cu and Ni are included in a large quantity, the specific gravity increases and as the weight increases, a merit that the strength is enhanced is conversely lost. Besides, a bulky crystallized substance causes a defect in forging and remarkably deteriorates the strength.

Further, other examples of an aluminum alloy for a forged piston may be employed, such as A4032 and A2618 according to JIS. However, as A4032 is material depending upon precipitation hardening, the strength of the piston is deteriorated when the material is exposed to high temperature. In the meantime, as only a small quantity of Si is included in A2618, the coefficient of thermal expansion increases and the sliding performance of the piston is deteriorated.

SUMMARY

In view of the above conventional designs, it is an object of the following disclosure to provide a forged piston with excellent properties in heat resistance and abrasion resistance and made of a light aluminum alloy.

In one embodiment, a forged piston comprises an aluminum alloy which includes 8 to 18 wt % of Si (wt %: percentage by weight), 0.5 to 3 wt % of Cu and 1 to 5 wt % of Ni, which further includes Al and unavoidable impurities, in which more Ni is included than Cu and in which the maximum length of an intermetallic compound generated by Al and Ni is 3 to 100 μm.

In another embodiment, a forged piston comprises an aluminum alloy which includes 8 to 18 wt % of Si, 0.5 to 3 wt % of Cu, 1 to 5 wt % of Ni and 2.0 wt % or less of Fe, which further includes Al and unavoidable impurities, in which the total in wt % of Ni and Fe is more than Cu and is equivalent to 5 wt % or less and in which the maximum length of an intermetallic compound generated by Al, Ni and Fe is 3 to 100 μm.

In yet another embodiment, the aluminum alloy further includes 2.0 wt % or less of Mg.

In another embodiment, the aluminum alloy further includes at least either of 0.25 wt % or less of Zr, or 0.25 wt % or less of Ti, and the total of Zr and Ti in the case where Zr and Ti are included is 0.3 wt % or less.

In another embodiment, the aluminum alloy further includes 2.0 wt % or less of at least one type of the total of Fe and Mn, and 2.0 wt % or less of the total of Fe and Cr×5.

In yet another embodiment, comprises a mean particle diameter of eutectic Si and pro-eutectic Si in the metallographic structure of the aluminum alloy is 10 to 100 μm.

As described above, a forged piston can be provided with excellent properties in heat resistance and abrasion resistance and made of the light aluminum alloy can be acquired by the aluminum alloy the strength at high temperature of which is enhanced, securing toughness and productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of one embodiment of a forged piston according to the inventive principles.

FIG. 2 illustrates a graph showing the results of measurement in which each fatigue strength of a forged piston according to the principles of the invention and conventional type forged pistons is measured.

FIG. 3 illustrates a photograph showing metallographic structure in the forged piston according to the principles of the invention.

FIG. 4 shows a photograph showing metallographic structure in a forged piston in a comparative example.

DETAILED DESCRIPTION

Referring to the drawings, a forged piston to which the inventive principles are applied will be described in detail below.

The forged piston for example, as shown in FIG. 1, has a structure provided with a piston head 1 for receiving an explosion load in a cylinder of an internal combustion engine, a skirt 2 for smoothly reciprocating the piston in the cylinder, a plurality of piston-ring grooves 3 for incorporating a piston ring which slidingly contacts an inner wall of the cylinder in the piston and a pin hole 4 for inserting a piston pin to connect the piston to a connecting rod.

The forged piston to which the inventive concepts may be applied is characterized in that it is made of an aluminum alloy which includes 8 to 18 wt % of Si, 0.5 to 3 wt % of Cu and 1 to 5 wt % of Ni, which further includes Al and unavoidable impurities, in which more Ni is included than Cu and in which the maximum length of an intermetallic compound generated by Al and Ni is 3 to 100 μm.

Further, the forged piston to which the inventive concepts may be applied is characterized in that it is made of an aluminum alloy which includes 8 to 18 wt % of Si, 0.5 to 3 wt % of Cu, 1 to 5 wt % of Ni and 2.0 wt % or less of Fe, which further includes Al and unavoidable impurities, in which the total in wt % of Ni and Fe is more than Cu and is equivalent to 5 wt % or less and in which the maximum length of an intermetallic compound generated by Al, Ni and Fe is 3 to 100 μm.

In the forged piston to which the inventive concepts may be applied, a coefficient of linear expansion of the piston reciprocated in the high-temperature cylinder is inhibited by Si and its dimensional change can be minimized. In addition, sliding resistance at high speed can be enhanced.

Moreover, in the forged piston to which the inventive concepts may be applied, the material strength at room temperature to 200° C. can be enhanced by Cu.

Further, in the forged piston to which the inventive concepts may be applied, the aluminum alloy crystallizes an Al—Cu—Ni ternary compound and enhances material strength up to 250° C.

Furthermore, in the forged piston to which the inventive concepts may be applied, an Al—Ni compound in the aluminum alloy enhances strength up to high temperature of 250° C. or higher, inhibits the generation of a bulky crystallized substance, and prevents an internal defect and a crack in forging.

Besides, in the forged piston to which the inventive concepts may be applied, the Al—Ni compound in the aluminum alloy can enhance the strength even when exposed to high temperatures of 250° C. or higher, can inhibit the generation of a bulky crystallized substance, and can prevent an internal defect and an occurrence of a crack caused by forging.

Therefore, the forged piston to which the inventive concepts may be applied can be made into a forged piston of light aluminum alloy with excellent properties in heat resistance and abrasion resistance by the aluminum alloy, the strength of which at high temperatures can be enhanced, while securing toughness and productivity.

In another embodiment, in the forged piston to which the inventive concepts may be applied, Mg can be further included in the aluminum alloy. In this case, the percentage content of Mg shall be 2.0 wt % or less. Hereby, the strength of the material can be enhanced by the strengthening of solid solution.

Further, in the forged piston to which the inventive concepts may be applied, at least either of Zr or Ti can be included in the aluminum alloy. In this case, the percentage content of Zr would be 0.25 wt % or less and that of Ti would be 0.25 wt % or less. Further, in the case where Zr and Ti are both included, the total of Zr and Ti would be 0.3 wt % or less. Hereby, a crystal grain can be made minute and the durability can be enhanced, while inhibiting the generation of a bulky crystallized substance.

Furthermore, in the forged piston to which the inventive concepts may be applied, at least either of Mn or Cr can be included in the aluminum alloy. In this case, as for the percentage content of Mn, the total of Fe and Mn would be 2.0 wt % or less and as to the percentage content of Cr, the total of Fe and Cr×5 would be 2.0 wt % or less, where “Cr×5” is five times a wt % of Cr. Hereby, in a range in which no bulky compound is generated, the metallographic structure can be made minute and the malleability can be enhanced.

Moreover, in the forged piston to which the inventive concepts may be applied, the particle size of eutectic Si and pro-eutectic Si in the metallographic structure of the aluminum alloy can be 10 to 100 μm. Hereby, sufficient toughness and productivity are secured and the sliding performance of the piston can be enhanced.

Descriptions as to why each element may be added to the aluminum alloy for making the forged piston, their added amounts, and the metallographic structure of the aluminum alloy are provided below.

(Si: 8 to 18 wt %)

Si is an element that can inhibit the linear expansion of the piston used at high temperature, minimize its dimensional change and resist sliding at high speed. However, when Si is added in a large quantity, bulky pro-eutectic Si is crystallized, and the toughness and the workability of the material are deteriorated. Therefore, in the invention, to acquire sufficient effect, its lower limit is 8 wt % and to secure sufficient toughness and workability as a piston, its upper limit is 18 wt %.

(Cu: 0.5 to 3 wt %)

Cu is an element for enhancing material strength between room temperature to approximately 200° C. by the strengthening of solid solution. However, when Cu is added in a large quantity, the crystallization of the Al—Ni compound is hindered, the enhancement of material strength at 250° C. or higher temperatures is hindered, and a light piston cannot be acquired because the specific gravity of the material increases.

Therefore, to acquire strength minimally required at 200° C., the lower limit of Cu is 0.5 wt % and to inhibit the increase of the specific gravity without preventing the crystallization of the Al—Ni compound, its upper limit is 3 wt %.

(Ni: 1 to 5 wt %)

Ni is an element for crystallizing the Al—Cu—Ni ternary compound by coexisting with Cu and enhancing strength at high temperature. The effect can act up to 250° C. However, to enhance strength up to 250° C. or higher temperatures, the Al—Ni compound is required to be crystallized. To crystallize the Al—Ni compound, Ni is required to be added in a larger quantity than Cu. However, when Ni is added in a large quantity, a crystallized substance is made bulky, the toughness of the material is deteriorated, which causes cracking and defects in forging. Therefore, to crystallize the Al—Ni compound required for strength at high temperature, the lower limit of Ni is 1 wt % and to inhibit the crystallization of a bulky compound, its upper limit is 5 wt %. Further, Ni is added in a larger quantity than Cu.

(Fe: 2.0 wt % or less)

Fe is an element for crystallizing a compound with Al like Ni and enhancing strength at high temperature and can be arbitrarily added, however, to inhibit bulking a crystallized substance, Fe equivalent to 2 wt % maximum can be included. As the Al—Ni—Fe compound is crystallized in the case where Fe and Ni are simultaneously added, the total of Ni and Fe would be 5 wt % or less. Further, the total of Ni and Fe would be more than Cu.

(Mg: 2.0 wt % or less)

Mg is an element for enhancing the strength of the material by the strengthening of solid solution and can be arbitrarily added, however, as the toughness of the material is deteriorated by the crystallization of Mg₂Si when Mg is added in a large quantity, a maximum of 2.0 wt % of Mg would be included.

(Zr: 0.25 wt % or less)

(Ti: 0.25 wt % or less)

Zr and Ti are both elements that contribute to making a crystal grain minute and enhancing heat resistance. However, when Zr and Ti are added in a large quantity, either or both crystallize a bulky compound with Al. Therefore, the percentage of Zr and Ti included would be respectively 0.25 wt % maximum to inhibit the generation of a bulky crystallized substance. The total of Zr and Ti when they are both included would be 0.3 wt % or less.

(Mn: total of Fe and Mn: 2.0 wt % or less)

(Cr: total of Fe and Cr×5: 2.0 wt % or less)

Mn and Cr are elements for enhancing strength at high temperature and making the metallographic structure minute, however, when they are added at the same time as Fe, a bulky compound is generated together with Al and Fe and the malleability is deteriorated. Therefore, an upper limit for the total of these Mn, Cr and Fe is preferred. That is, the total of Fe and Mn would be 2.0 wt % or less and the total of Fe and Cr×5 would be 2.0% less.

Next, the metallographic structure of the aluminum alloy will be described.

(Intermetallic Compound)

An intermetallic compound of Al, Ni and a transition element such as Fe enhances material strength up to high temperature by inhibiting the transformation of material. However, as the effect of the intermetallic compound decreases when the intermetallic compound is made too minute, a lower limit of the maximum length is 3 μm. In the meantime, as cracking and defects are caused in forging when the intermetallic compound is made too excessive, an upper limit of the maximum length is 100 μm so that the intermetallic compound can be resistant to forging.

(Eutectic Si, Pro-Eutectic Si)

Eutectic Si and pro-eutectic Si are effective to enhance sliding performance, however, when the particle sizes of them are made too excessive, cracking and defects are caused in forging. Therefore, a lower limit of the particle sizes of eutectic Si and pro-eutectic Si would be 3 μm to gain sufficient effect and an upper limit would be 100 μm to resist forging.

The effects of the invention are detailed by exemplary embodiments below. The invention is not limited to the following embodiments and can be suitably changed in a range in which the outline is not changed.

First, an aluminum alloy including each additional element shown in Table 1 is forged into the shape of a piston and forged pistons of samples 1 to 12 are produced. As for the forged piston of each sample 1 to 12, a mechanical characteristic and malleability are evaluated. The samples 1 to 5 of these are the embodiments that meet conditions of the inventive principles and the samples 6 to 10 are comparative examples that do not meet the conditions of the inventive principles. The sample 11 shows JIS A4032 and the sample 12 shows JIS AC8A.

As for mechanical characteristics out of the results of evaluation shown in Table 1, each test piece is cut from the forged piston of each sample 1 to 12, after it is kept at 300° C. for 100 hours, a tension test is made in an atmosphere of 300° C., and the tensile strength and the extension of each test piece are measured. As for malleability, each sample 1 to 12 is heated at 350° C. or higher and it is checked whether a crack is made or not in forging the shape of the piston. In Table 1, ∘ shows a case that no crack is made and × shows a case that a crack is made. TABLE 1 Tensile Compound Strength Extension Size Specific No Si Cu Ni Fe Mg Mn Cr Zr Ti (Mpa) (%) (μm) Malleability Gravity Embodiment 1 12 1 3 — 1 — — — — 72 21 10-30 ◯ 2.71 2 12 1 3 — 1 1 — — — 81 17 10-40 ◯ 2.73 3 12 1 3 — 1 — −0.2 — — 78 18 10-20 ◯ 2.71 4 12 1 2 0.6 0.5 — — 0.1 0.1 73 24  3-30 ◯ 2.71 5 12 1 2 — 1 — — 0.1 0.1 71 23  3-30 ◯ 2.69 Comparative 6 12 5 5 — 1 — — — — 84 15 100< X 2.83 Example 7 12 5 3 — 1 — — — — 80 22 10-30 ◯ 2.79 8 12 1 3 1   1 — — — — 78 16 100< X 2.73 9 12 1 2 0.6 1 0.5 — 0.1 0.1 84 19 100< X 2.71 10 12 1 2 0.6 1 —   0.2 — 0   81 18 100< X 2.70 A4032 11 12 1 1 — 1 — — — — 60 35 — ◯ 2.67 AC8A 12 12 1 1 — 1 — — — — 76 15 — — 2.67

The results of evaluation shown in Table 1 show that the samples 1 to 6 (the embodiments) higher tensile strength than A4032 and show larger extension than AC8A. Besides, the samples 1 to 6 have no problem in relation to malleability and also show no increase in specific gravity to the extension that the specific gravity has an effect upon each actual piston. Further, the embodiments 2 and 3 show higher tensile strength and larger extension than AC8A. In the meantime, the embodiments 1, 4 and 5 show higher strength than AC8A in relation to fatigue strength which is important in each actual piston and owing to the enhancement of each internal quality by minute metallographic structure and malleability, though each tensile strength is inferior to that in AC8A.

In the meantime, in the samples 6, 8, 9 and 10 (the comparative examples), though they show a high value in relation to mechanical strength, a crack is caused in forging in any case. Tests of the metallographic structure of these samples 6, 8, 9 and 10 show that in the comparative example 6, Al—Ni bulky compound is crystallized, in the comparative example 8, an Al—Ni—Fe bulky compound is crystallized, in the comparative example 9, an Al—Ni—Fe—Mn bulky compound is crystallized, and in the comparative example 10, an Al—Ni—Fe—Cr bulky compound is crystallized. These bulky compounds exceed 100 μm in length in any case.

JP-A No. 54053/2000 shows the effect of micronizing a crystallized substance by Mn and shows that the maximum length of the crystallized substance is reduced by changing the shape of the crystallized substance from a sharp and narrow shape to a lump. However, in the reference, the volume itself of an individual crystallized substance is not reduced and the maximum length is also at a level of 100 μm. Therefore, the effect of micronization to the extent that malleability is enhanced, cannot be recognized as in the presently disclosed inventive concepts.

In the sample 7 (the comparative example), the strength is high, no bulky crystallized substance is caused, and the malleability is also satisfactory. However, the specific gravity is larger by approximately 5%, compared with that in conventional type A4032 and AC8A. This is contrary to the object of the invention of manufacturing the light piston. In the meantime, in the samples 1 to 6 (the embodiments), each specific gravity is 2.75 or less and further lightening is enabled by inhibiting the increase of the specific gravity, maintaining high strength resistant to high temperature and excellent malleability.

Next, a fatigue strength test is made for the forged pistons in the sample 4 (the invention), the sample 11 (A4032) and the sample 12 (AC8A). In this fatigue strength test, a fatigue curve under pulsating tension near to stress generated in each actual piston in an atmosphere of 300° C. is measured. FIG. 2 shows the results of the measurement.

The results of the measurement shown in FIG. 2 show that in the sample 4, though the tensile strength is lower than that in AC8A, the fatigue strength is higher by 10% or more than that in AC8A. The results also show that the fatigue strength in the sample 4 is higher by 40% or more than that in A40432 and that the strength of sample 4 is greatly enhanced.

Next, FIG. 3 shows a photograph of a microstructure showing the forged piston in the sample 4 (the invention) and FIG. 4 shows a photograph of a microstructure in the sample 9 (the comparative example). These photographs of the microstructures shown in FIGS. 3 and 4 show that in the forged piston in the comparative example, the maximum length of an intermetallic compound in metallographic structure is 100 μm or longer, while in the forged piston according to the present inventive concepts, the intermetallic compounds in the metallographic structure are minutely and homogeneously dispersed and are controlled at a level at which the malleability is not hindered. 

1. A forged piston made of an aluminum alloy, comprising: 8 to 18 wt % of Si; 0.5 to 3 wt % of Cu; and 1 to 5 wt % of Ni, further comprising: Al; and unavoidable impurities, wherein: more Ni is included than Cu; and the maximum length of an intermetallic compound generated by Al and Ni is 3 to 100 μm.
 2. A forged piston made of an aluminum alloy, comprising: 8 to 18 wt % of Si; 0.5 to 3 wt % of Cu; 1 to 5 wt % of Ni; and 2.0 wt % or less of Fe, further comprising: Al; and unavoidable impurities, wherein: the total of Ni and Fe is more than Cu and is equivalent to 5 wt % or less; and the maximum length of an intermetallic compound generated by Al, Ni and Fe is 3 to 100 μm.
 3. A forged piston according to claim 1, wherein: the aluminum alloy further includes 2.0 wt % or less of Mg.
 4. A forged piston according to claim 1, wherein: the aluminum alloy further includes at least either of 0.25 wt % or less of Zr, or 0.25 wt % or less of Ti; and the total of Zr and Ti in the case where Zr and Ti are both included is equivalent to 0.3 wt % or less.
 5. A forged piston according to any of claim 1, wherein: the aluminum alloy further includes at least either of 2.0 wt % or less of the total of Fe and Mn, or 2.0 wt % or less of the total of Fe and Cr×5.
 6. A forged piston according to any of claim 1, wherein: the particle size of eutectic Si and pro-eutectic Si in metallographic structure of the aluminum alloy is 10 to 100 μm.
 7. A forged piston according to claim 2, wherein: the aluminum alloy further includes 2.0 wt % or less of Mg.
 8. A forged piston according to claim 2, wherein: the aluminum alloy further includes at least either of 0.25 wt % or less of Zr, or 0.25 wt % or less of Ti; and the total of Zr and Ti in the case where Zr and Ti are both included is equivalent to 0.3 wt % or less.
 9. A forged piston according to any of claim 2, wherein: the aluminum alloy further includes at least either of 2.0 wt % or less of the total of Fe and Mn, or 2.0 wt % or less of the total of Fe and Cr×5.
 10. A forged piston according to any of claim 2, wherein: the particle size of eutectic Si and pro-eutectic Si in metallographic structure of the aluminum alloy is 10 to 100 μm. 